1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus in which an electrostatic latent image is formed on an image carrier with a electrophotographic process or an electrostatic recording process, for example, and then developed using a two-component developer. Further, the present invention relates to a process cartridge and an electrophotographic image forming apparatus to which the process cartridge is mounted in a detachable manner.
Herein, the terms xe2x80x9cimage forming apparatusxe2x80x9d and xe2x80x9celectrophotographic image forming apparatusxe2x80x9d include, for example, copying machines, printers (such as an LED printer and a laser beam printer), facsimiles, word processors, etc.
Also, the term xe2x80x9cprocess cartridgexe2x80x9d implies not only an integral unit comprising at least one of a charging means, developing means, and cleaning means, and an electrophotographic photoconductor, which are constructed in the form of a cartridge, the cartridge being detachably mounted to a body of an electrophotographic image forming apparatus, but also implies an integral unit comprising at least a developing means and an electrophotographic photoconductor which are constructed in the form of a cartridge, the cartridge being detachably mounted to the body of the electrophotographic image forming apparatus.
2. Description of the Related Art
Hitherto, the so-called two-component developing method has been employed in many apparatuses. The two-component developing method utilizes a two-component developer, which is made up of a nonmagnetic toner (hereinafter referred to simply as a xe2x80x9ctonerxe2x80x9d) and a magnetic carrier (hereinafter referred to simply as a xe2x80x9ccarrierxe2x80x9d), as a developer to visualize an electrostatic latent image formed on an image carrier.
According to the two-component developing method, a two-component developer is agitated by a developer agitating and transporting means (hereinafter referred to simply as an xe2x80x9cagitating meansxe2x80x9d) disposed in a developer container for containing the developer, so that the toner is electrically charged as a result of friction. Thereafter, the developer is transported to a developing sleeve that serves as a developer carrier and includes a stationary magnet roller disposed therein. Then, the developer is borne on the surface of the developing sleeve and transported with the rotation of the developing sleeve. The developer is thereby supplied to an electrostatic latent image formed on an image carrier for developing the latent image.
A two-component developing device employing the two-component developing method is advantageous in that it has a longer useful life and a lower operating cost because the device can be used repeatedly by replenishing only the toner from a toner resupply unit provided separately. Accordingly the two-component developing device has been widely used.
A process cartridge scheme is also known in which an electrophotographic photoconductor and at least one process means acting on the electrophotographic photoconductor are constructed into an integral unit in the form of a cartridge, and the cartridge is detachably mounted to a body of an image forming apparatus. This process cartridge scheme enables a user to carry out maintenance of the apparatus without the need for a serviceman, and hence can achieve a noticeable improvement in operability. For that reason, this process cartridge scheme has been widely used in electrophotographic image forming apparatuses.
FIG. 7 schematically shows a cross-section of a typical conventional two-component developing device. A developer container 50 for accommodating a two-component developer includes, as a developer carrier, a developing sleeve 51 rotatable in the direction of arrow xe2x80x9cyxe2x80x9d. The developing sleeve 51 is a hollow metallic sleeve including a magnet roller 52 disposed therein, which serves as a magnetic field generating means. As shown in FIG. 7, a doctor blade 53 serving as a developer layer thickness restricting means is provided closely below the developing sleeve 51. As the developing sleeve 51 rotates in the direction of arrow xe2x80x9cyxe2x80x9d, the developer transported to a gap between the developing sleeve 51 and the doctor blade 53 is formed into a thin layer by the doctor blade 53.
In the developer container 50, an A screw 54 is disposed as a first agitating means to extend substantially parallel with the longitudinal direction of the developing sleeve 51. As the A screw rotates in the direction of the arrow xe2x80x9cxxe2x80x9d in FIG. 7, it transports and agitates the developer. On the opposite side of the developing sleeve 51 to the A screw 54, a B screw 55 is disposed as a second agitating means rotatably in the direction of arrow xe2x80x9czxe2x80x9d in FIG. 7.
Also, a toner density sensor 56 serving as a developer amount detecting means is provided on a wall surface of the developer container 50 which faces the B screw 55 on the side opposite to the A screw 54. A sensor surface 56a of the toner density sensor 56 is positioned near the B screw 55 and arranged to lie perpendicularly to a line connecting a rotary shaft of the B screw 55 and the toner density sensor 56. The reason why the sensor surface 56a is so arranged with respect to the B screw 55 is for preventing buildup of the developer on the sensor surface 56a. If the toner is built up on the sensor surface 56a, the toner density sensor 56 will fail to precisely detect a toner density (i.e., a mixing ratio of the carrier and the toner) in the developer.
FIG. 8 schematically shows the construction of the developing device of FIG. 7 as viewed from above. The A screw 54 and the B screw 55 are arranged substantially parallel with each other, and an inner wall 57 is provided between both the screws 54, 55 as a partition to prevent the developer from moving directly from one of both screws 54, 55 to the other. However, the inner wall 57 is not provided in areas corresponding to longitudinal opposite portions of the A screw 54 and the B screw 55, allowing the developer to move between both screws 54, 55. The A screw 54 and the B screw 55 are rotated in the directions of their respective arrows shown in FIG. 7 to transport the developer in the opposite longitudinal directions, i.e., in the directions of respective arrows X and Y shown in FIG. 8. Thus, a circulation path for the developer is formed within the developer container 50 to ensure continuous developer circulation.
The toner density sensor 56 is disposed in the upstream side of the B screw 55 in the direction of transport of the developer. The reason for arranging the toner density sensor 56 in the upstream side of the B screw 55 in the direction of transport of the developer is for enabling toner density detection to be immediately made on the developer that has been subjected to image formation using the toner and has a reduced toner density. More specifically, the developer residing in the A screw 54 side (hereinafter referred to as a xe2x80x9cdevelopment chamber 50Axe2x80x9d) of the developer container 50 partitioned by the inner wall 57 is borne by the developer carrier and employed for image formation. Thereafter, the developer is sent to the B screw 55 side (hereinafter referred to as an xe2x80x9cagitation chamber 50Bxe2x80x9d) of the developer container 50 partitioned by the inner wall 57 following the above-described circulation path, and the toner density is immediately detected by the toner density sensor 5. In accordance with a detected result, an appropriate amount of the developer is replenished from a toner resupply unit 59 (FIG. 7), which is provided adjacent to the developer container 50 and communicated with it, through a toner resupply port 58 positioned downstream of the toner density sensor 56 in the direction of transport of the developer. Consequently, the toner density of the developer is kept constant.
As seen from the above, the circulation of the developer is particularly important in the two-component developing device.
The above-described conventional developing device, however, has a problem that it is sometimes difficult to circulate the developer in a satisfactory manner depending on long-duration operation and environmental conditions.
Stated otherwise, the following points must be taken into consideration to realize satisfactory circulation of the developer in the two-component developing device.
First, the position of an upper surface of the developer (hereinafter referred to as a xe2x80x9cdeveloper surfacexe2x80x9d) contained in the A screw 54 side, i.e., in the development chamber 50A, is preferably high to some extent. If the developer surface is lowered down beyond a certain level, the total amount of the developer transported by the a screw 54 would be so small that the amount of the developer supplied to the developing sleeve 51 and retained by the doctor blade 53 is reduced and variations in the supply of the developer from the A screw 54 to the developing sleeve 51 tend to occur in corresponding areas. More specifically, variations in the supply of the developer are likely to occur corresponding to the pitch of a spiral vane 54a of the A screw 54, and a coating of the developer formed on the developing sleeve 51 is apt to have thicker and thinner portions at the pitch of the vane 54a. This causes the so-called screw pitch unevenness, i.e., unevenness in image density at the screw pitch. For that reason, it is desired that the level of the developer surface in the development chamber 50A be high.
Secondly, the position of an upper surface of the developer (i.e., a developer surface) contained in the B screw 55 side, i.e., in the agitation chamber 50B, is preferably lower than the top of a spiral vane 55a of the B screw 55. The reason is that the agitation chamber 50B has a function of agitating the developer, and if the developer surface is too high, there would occur a difficulty in agitating the developer residing at a level higher than the top of the vane 55a. Particularly, as shown in FIG. 9, if the toner is replenished under a condition where the level of the developer surface is higher than the top of the vane 55a, a newly added toner having a smaller specific gravity than the developer will often remain floating on the developer surface. Such a phenomenon causes problems that the toner is hard to evenily mix with the developer and the toner not yet electrically charged is supplied to the development chamber 50A, thus resulting in fogging or density failure. Where the level of the developer surface in the agitation chamber 50B is lower than the top of the B screw 55, the replenished toner is forced to be taken into the developer with the rotation of the B screw 55 and then sufficiently agitated, thus not giving rise to problems, such as fogging and density failure.
Thirdly, the position of the developer surface in the agitation chamber 50B is preferably higher than the sensor surface 56a of the toner density sensor 56. If the sensor surface 56a is not covered with the developer, the toner density sensor 56 would generate a noticeably dropped sensor output and detect that the toner density is very small. Even with the developer surface level being lower than the top of the sensor surface 56a of the toner density sensor 56, a sensor output value will not vary if the developer surface level is kept constant with stability. In practice, however, the developer surface level varies to some extent. Accordingly, when the sensor surface 56a is not completely covered with the developer, variations in sensor output are remarkably increased, thus resulting in an unsatisfactory result. One conceivable method for overcoming the above-described problem is to mount the sensor surface 56a at a lower position, but there is actually a restriction in selecting the mount position of the sensor surface 56a from the relationship in size between the developer container 50 and the sensor surface 56a. 
Fourthly, the developer surfaces in the development chamber 50A and the agitation chamber 50B are preferably almost horizontal in each of the development chamber 50A and the agitation chamber 50B. If the developer surface in the development chamber 50A is inclined in the longitudinal direction, the amount of the developer supplied to the developing sleeve 51 would be uneven in the longitudinal direction and a formed image would have a density difference in the corresponding direction. Also, if the developer surface in the agitation chamber 50B is inclined, a charging capability as a result of friction would be deteriorated.
For satisfying the above first to third conditions, it is conceivable to adjust the pitches of the screw vanes 54a, 55a and the rotational speeds of the A and B screws 54, 55 to relatively increase transport power of the B screw so that the developer surface in the A screw 54 side (the development chamber 50A) is kept higher and the developer surface in the B screw 55 side (the agitation chamber 50B) is kept lower. However, this method will raise the developer surface level in the upstream side of the development chamber 50A in the direction of transport of the developer and lower the developer surface level on the downstream side thereof. Eventually, the method cannot satisfy the above-described fourth condition and gives rise to a density difference in the longitudinal direction.
For satisfying the above-described first to fourth conditions, it is therefore required to optimize the developer surface levels in the development chamber 50A and the agitation chamber 50B while holding the developer surfaces in both chambers at respective levels not much different from each other and maintaining stable circulation of the developer, and to optimize the amount of the developer for realizing the optimum developer surface levels.
However, the bulk density of the two-component developer changes depending on environmental conditions and a long-duration operation, and such a change brings about variations in level of the developer surface in the developer container 50. This phenomenon is presumably attributable to variations in the amount of electricity charged on the toner. It is generally known that the developer surface level is raised in a low humidity environment and lowered in a high humidity environment.
Thus, even when the amount of the developer is held optimum to closely control the developer surface level in the developer container 50, the developer surface level eventually varies for the reasons such as environmental conditions and a long-duration operation. In other words, controlling the developer surface level in a proper range to satisfy the above-mentioned first to fourth conditions has been hitherto impossible to realize in practice.
The present invention has been accomplished in view of the above-described problems in the related art.
Accordingly, it is an object of the present invention to provide a developing device and an image forming apparatus, which can always produce a high-quality image.
Another object of the present invention is to provide a developing device and an image forming apparatus, which can always produce a high-quality image by holding the position of an upper surface of a developer (i.e., a developer surface), accommodated in a developer container, at an appropriate level to prevent image failures such as unevenness in image density and fogging.
Still another object of the present invention is to provide a developing device and an image forming apparatus, which can maintain the level of the developer surface in a appropriate range to prevent image failures, such as unevenness in image density and fogging, with stability even when the bulk density of the developer in the developer container changes depending on environmental conditions and a long-duration operation, and which can always operate with stable performance.
Still another object of the present invention is to provide a developing device and an image forming apparatus, each of which comprises a developer container for containing a developer made up of a magnetic particle and a toner; a developer carrier for bearing and transporting the developer; a first agitating and transporting unit provided in the developer container, the first agitating and transporting unit transporting the developer while agitating the developer; a second agitating and transporting unit provided in the developer container, the second agitating and transporting unit transporting the developer transported by the first agitating and transporting unit while agitating the developer, thereby supplying the developer to the developer carrier; and a restricting member provided in the developer container, the restricting member restricting passage of an upper portion of the developer transported by the first agitating and transporting unit, thereby maintaining a desired developer surface level.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.